In Third Degree AV Block, what is the relationship between atrial and ventricular activity?

• A. P waves always precede QRS with constant PR interval
• B. P waves and QRS complexes show complete dissociation, with no consistent AV conduction
• C. QRS follows P waves with preserved AV conduction
• D. Ventricular rate identical to atrial rate due to pacing

Answer: (B)

This question tests recognition of complete AV dissociation in third-degree block. In this condition, the electrical impulses from the atria and the ventricles are completely independent because conduction from the atria to the ventricles is blocked. The atria beat at their own intrinsic rate driven by the SA node, while the ventricles are paced by an intrinsic escape rhythm below the block. Because there’s no conduction between the two, there’s no fixed relationship between P waves and QRS complexes or a constant PR interval.

That’s why the correct description is that P waves and QRS complexes show complete dissociation, with no consistent AV conduction. The atrial rate is typically faster than the ventricular rate, and the two pedigrees run independently.

The other descriptions imply some intact AV conduction or a direct pacing link between atrial and ventricular activity, which isn’t present in third-degree block. In practice, this rhythm is a medical emergency and often requires urgent pacing.